© 2010 pearson education, inc. how does light tell us the temperatures of planets and stars?

© 2010 Pearson Education, Inc.

How does light tell us the temperatures of planets and stars?

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Thermal Radiation

• Nearly all large or dense objects emit thermal radiation, including stars, planets, you.

• An object’s thermal radiation spectrum depends on only one property: its temperature.

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Properties of Thermal Radiation1. The hotter the object, the more energy is emitted at

all wavelengths – Stefan-Boltzmann’s Law

2. The hotter the object, the shorter the peak of maximum emission – Wien’s Law

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Clicker QuestionWhich is hottest?

a) a blue star

b) a red star

c) a planet that emits only infrared light

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Clicker QuestionWhich is hottest?

a) a blue star

b) a red star

c) a planet that emits only infrared light

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Clicker QuestionWhy don’t we glow in the dark?

a) People do not emit any kind of light.b) People only emit light that is invisible to our

eyes.c) People are too small to emit enough light for us

to see. d) People do not contain enough radioactive

material.

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Clicker QuestionWhy don’t we glow in the dark?

a) People do not emit any kind of light.b) People only emit light that is invisible to our

eyes.c) People are too small to emit enough light for us

to see. d) People do not contain enough radioactive

material.

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How do we interpret an actual spectrum?

• By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

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What is this object?

Reflected sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed—object must look red.

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What is this object?

Thermal radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K.

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What is this object?

Carbon dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere.

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What is this object?

Ultraviolet emission lines: Indicate a hot upper atmosphere

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Clicker QuestionWhat is this object?

a) Neptune

b) Jupiter

c) The sun

d) Mars

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Clicker QuestionWhat is this object?

a) Neptune

b) Jupiter

c) The sun

d) Mars

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How does light tell us the speed of a distant object?

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Measuring the Shift

• We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

Stationary

Moving away

Away faster

Moving toward

Toward faster

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Doppler shift tells us ONLY about the part of an object’s velocity toward or away from us, the radial velocity.

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Clicker Question

a) It is moving away from me.

b) It is moving toward me.

c) It has unusually long spectral lines.

I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star?

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Clicker Question

a) It is moving away from me.

b) It is moving toward me.

c) It has unusually long spectral lines.

I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star?

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Spectrum of a Rotating Object

• Spectral lines are wider when an object rotates faster.